This invention relates to increasing the efficiency of utilization of input, output, and/or input/output pins on integrated circuits such as programmable logic devices. (For ease of reference, all pins--whether input, output, or input/output--are generally referred to generically herein as input/output or I/O pins. Also for ease of reference, multiplexing and demultiplexing of signals are both sometimes referred to simply as multiplexing.)
It has been proposed to increase data throughput through I/O pins of integrated circuits by time-division multiplexing data flowing through those pins. Sample et al. U.S. patent 6,020,760, for example, shows integrated circuits such as programmable logic devices in which I/O pins can optionally transmit and/or receive different data signal values in association with each half of each cycle of the circuit's basic clock signal. This is twice the normal data rate, in which only one data signal value is transmitted or received during each full clock signal cycle. Accordingly, the Sample et al. reference shows increased pin utilization efficiency, which can help to alleviate possible shortages of pins in dense integrated circuits. (The Sample et al. reference is hereby incorporated by reference herein in its entirety.)
Circuitry of the type shown in the above-mentioned Sample et al. reference may add signal switching (multiplexing) to circuit paths that may sometimes be needed for speed-critical signals. Such additional multiplexing tends to add delay and may therefore be undesirable for possibly speed-critical circuit paths.
In view of the foregoing, it is an object of this invention to provide improved circuitry for optionally multiplexing signals on I/O pins of integrated circuits.
It is a more particular object of this invention to provide circuitry which allows two data signals to be optionally multiplexed onto a single I/O pin without introducing additional delay into the I/O pin path, especially when the optional multiplexing is not in use and only a single signal is to pass through the I/O pin.